The present invention relates to a power supply apparatus and to a method for controlling current utilizing the apparatus.
For a small capacity power supply apparatus for supplying DC power to electronic apparatuses, such as a sensor for transmission or distribution of electrical energy, a through-type current transformer has been used, which allows clipping the primary side on the main circuit to obtain electrical energy from a strong current circuit (main circuit) without requiring electric work. Also, a DC power supply apparatus of a current transformer input type has been widely used which has a configuration in which the primary side is clipped on the main circuit and the voltage generated in the secondary winding is rectified and smoothed to output a DC voltage.
In the case of a DC power supply apparatus of a current transformer input type as mentioned above, the winding ratio of a current transformer is needed to be able to supply a sufficient load current even at the minimum main circuit current. But, when dealing with transmission or distribution of electric energy, it is desirable to design the dynamic range of the main circuit current as wide as possible. Therefore, the change or increment exceeding the minimum main circuit current becomes an excess input to the current transformer, resulting in circuit loss in the power supply circuit. For example, if the dynamic range of the primary current (main circuit current) is designed to be 10 to 100 A, then when input current is 100 A, in the secondary side 90 A is excess power, which increases the loss of current and heat. Since the output voltage of the power supply apparatus is usually required to be constant in the load side, a circuit on the secondary winding of the current transformer has to bear the large product of VA at excess input. In other words, the current transformer has to be made large to compensate for the increased VA.
Conventionally, in order to decrease the loss in the secondary winding of the current transformer in the case of an excess current, the excess current state is detected by monitoring the rectified current and smoothed output on the secondary side is provided. Then, the secondary winding is short-circuited with the use of an AC switch like a Triac (triode AC switch) by utilizing a synchronous rectifier circuit of a low loss with the use of a field effect transistor.
However, an AC switch like Triac, has a PNPN junction, so that at the time of short circuit, a voltage drop for 2 diodes is generated and the loss becomes large. Also, the control circuit for the AC switch is complicated and costly. Also, a synchronous rectifier switch with the use of a field effect transistor has drawbacks as well. For instance, if a field effect transistor having a very small ON-resistance is used in the rectifier portion or in the AC switch in order to make the loss small, the secondary winding voltage necessary to drive the synchronous rectification also becomes small when short-circuiting. Therefore, it is difficult to realize a synchronous rectifier with high efficiency. In both cases, the circuit losses are not controlled sufficiently and a small sized current transformer is not realized.
The present invention provides a power supply apparatus of a current transformer input type having a small circuit loss and a wide input dynamic range. Also, the present invention provides a small sized power supply apparatus which can be clipped on the main circuit (live line).
In an object of the present invention an AC switch which is used to short-circuit the secondary side of a current transformer, is constituted with a switch having no offset voltage at the buildup of a current and has a low ON-resistance. The AC switch can preferably be constituted with a field effect transistor (MOS). A control signal for this AC switch uses accumulated charges in a smoothing circuit on the side of a DC power supply. When the output power of a secondary winding of a current transformer exceeds the necessary VA for the DC power supply (at the time of an excess input), the secondary winding of the current transformer is short-circuited with the AC switch allowing any excess input to be controlled and circuit loss to be decreased.